Field of the Invention
The present invention relates to a reflector filter operating with acoustic surface waves, having an input transducer, an output transducer and having a reflector structure. The signal is to proceed from the input transducer to the output transducer on an acoustic path via the reflector structure in order to obtain the required filtering effect. Direct signal transfer between the input transducer and the output transducer is undesired crosstalk, which is to be prevented as fully as possible. Known solutions to this problem are to arrange the input transducer and the output transducer on the substrate of the filter in such a way that the direct acoustic signal transfer from the input to the output is prevented. The opposite is the case with a resonator filter.
Reflector surface wave filters, referred to as RAC filters, are known from the U.S. Pat. Nos. 4,484,160 and 4,520,330. One embodiment comprises an input transducer and an output transducer and two reflector structures which have reflector fingers that are positioned obliquely in such a way that an acoustic wave proceeding into one reflector is deflected by approximately 90.degree. and proceeds into the other reflector and, in turn deflected 90.degree. by the other reflector, is supplied to the output transducer. A filter of this type is suitable as a broadband filter for use in defining a time interval. The finger structures of the aforesaid reflectors can be weighted and/or dispersive in accordance with the desired transmission function. In particular, finger displacement weighting, finger omission weighting and finger rotation can be provided as finger weighting. Such a known reflector filter has two acoustic tracks and an input transducer and an output transducer are located in one of the two tracks in each case. As a result of the folded acoustic path, such a filter requires a smaller length of the substrate, which also incurs substantial savings. The known RAC filter has however a certain disadvantage, in that the angle accuracy of the reflection is temperature-dependent, even in the case of a substrate made of quartz.
Further embodiments of multi-track reflector filters with multi-strip couplers are disclosed in IEEE Trans. Ultrasonics, Vol. 35 (1988), pages 61 ff. The multi-strip coupler requires a substrate with a high electroacoustic coupling factor. However, such substrates are temperature-dependent to a relatively high degree with respect to their wave propagation properties. Moreover, multi-strip couplers are susceptible to losses and also make it more difficult to calculate the behavior of such a filter.
A prior art reference that is only of apparent relevance to the invention to be described below which has surface wave filters having reflectors with 180.degree. reflection arranged in two tracks is known from "Patent Abstracts of Japan", Vol. 10, No. 363 (E-462) (2429), p. 1286. However, an independent filter is realized therein in each of the tracks. One track is a resonator filter that serves as a component of an oscillator and the other track is the output filter for the entire circuit. None of the two surface wave arrangements is a reflector filter as provided by the invention with useful signal transmission of the signal frequency provided only via the reflector.